PERLUNICODE

NAME

DESCRIPTION

Important Caveats

Unicode support is an extensive requirement. While Perl does not
implement the Unicode standard or the accompanying technical reports
from cover to cover, Perl does support many Unicode features.

People who want to learn to use Unicode in Perl, should probably read
the Perl Unicode tutorial before reading this reference
document.

Input and Output Layers

Perl knows when a filehandle uses Perl's internal Unicode encodings
(UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened with
the ``:utf8'' layer. Other encodings can be converted to Perl's
encoding on input or from Perl's encoding on output by use of the
``:encoding(...)'' layer. See open.

To indicate that Perl source itself is in UTF-8, use "use utf8;".

Regular Expressions

The regular expression compiler produces polymorphic opcodes. That is,
the pattern adapts to the data and automatically switches to the Unicode
character scheme when presented with data that is internally encoded in
UTF-8 --- or instead uses a traditional byte scheme when presented with
byte data.

"use utf8" still needed to enable UTF-8/UTF-EBCDIC in scripts

As a compatibility measure, the "use utf8" pragma must be explicitly
included to enable recognition of UTF-8 in the Perl scripts themselves
(in string or regular expression literals, or in identifier names) on
ASCII-based machines or to recognize UTF-EBCDIC on EBCDIC-based
machines. These are the only times when an explicit "use utf8"
is needed. See utf8.

BOM-marked scripts and UTF-16 scripts autodetected

If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE,
or UTF-8), or if the script looks like non-BOM-marked UTF-16 of either
endianness, Perl will correctly read in the script as Unicode.
(BOMless UTF-8 cannot be effectively recognized or differentiated from
ISO 8859-1 or other eight-bit encodings.)

"use encoding" needed to upgrade non-Latin-1 byte strings

By default, there is a fundamental asymmetry in Perl's Unicode model:
implicit upgrading from byte strings to Unicode strings assumes that
they were encoded in ISO 8859-1 (Latin-1), but Unicode strings are
downgraded with UTF-8 encoding. This happens because the first 256
codepoints in Unicode happens to agree with Latin-1.

See ``Byte and Character Semantics'' for more details.

Byte and Character Semantics

In future, Perl-level operations will be expected to work with
characters rather than bytes.

However, as an interim compatibility measure, Perl aims to
provide a safe migration path from byte semantics to character
semantics for programs. For operations where Perl can unambiguously
decide that the input data are characters, Perl switches to
character semantics. For operations where this determination cannot
be made without additional information from the user, Perl decides in
favor of compatibility and chooses to use byte semantics.

This behavior preserves compatibility with earlier versions of Perl,
which allowed byte semantics in Perl operations only if
none of the program's inputs were marked as being as source of Unicode
character data. Such data may come from filehandles, from calls to
external programs, from information provided by the system (such as %ENV),
or from literals and constants in the source text.

The "bytes" pragma will always, regardless of platform, force byte
semantics in a particular lexical scope. See bytes.

The "utf8" pragma is primarily a compatibility device that enables
recognition of UTF-(8|EBCDIC) in literals encountered by the parser.
Note that this pragma is only required while Perl defaults to byte
semantics; when character semantics become the default, this pragma
may become a no-op. See utf8.

Unless explicitly stated, Perl operators use character semantics
for Unicode data and byte semantics for non-Unicode data.
The decision to use character semantics is made transparently. If
input data comes from a Unicode source---for example, if a character
encoding layer is added to a filehandle or a literal Unicode
string constant appears in a program---character semantics apply.
Otherwise, byte semantics are in effect. The "bytes" pragma should
be used to force byte semantics on Unicode data.

If strings operating under byte semantics and strings with Unicode
character data are concatenated, the new string will be created by
decoding the byte strings as ISO 8859-1 (Latin-1), even if the
old Unicode string used EBCDIC. This translation is done without
regard to the system's native 8-bit encoding.

Under character semantics, many operations that formerly operated on
bytes now operate on characters. A character in Perl is
logically just a number ranging from 0 to 2**31 or so. Larger
characters may encode into longer sequences of bytes internally, but
this internal detail is mostly hidden for Perl code.
See perluniintro for more.

Effects of Character Semantics

If you use a Unicode editor to edit your program, Unicode characters may
occur directly within the literal strings in UTF-8 encoding, or UTF-16.
(The former requires a BOM or "use utf8", the latter requires a BOM.)

Unicode characters can also be added to a string by using the "\x{...}"
notation. The Unicode code for the desired character, in hexadecimal,
should be placed in the braces. For instance, a smiley face is
"\x{263A}". This encoding scheme only works for all characters, but
for characters under 0x100, note that Perl may use an 8 bit encoding
internally, for optimization and/or backward compatibility.

Additionally, if you

use charnames ':full';

you can use the "\N{...}" notation and put the official Unicode
character name within the braces, such as "\N{WHITE SMILING FACE}".

•

If an appropriate encoding is specified, identifiers within the
Perl script may contain Unicode alphanumeric characters, including
ideographs. Perl does not currently attempt to canonicalize variable
names.

•

Regular expressions match characters instead of bytes. ``.'' matches
a character instead of a byte.

•

Character classes in regular expressions match characters instead of
bytes and match against the character properties specified in the
Unicode properties database. "\w" can be used to match a Japanese
ideograph, for instance.

•

Named Unicode properties, scripts, and block ranges may be used like
character classes via the "\p{}" ``matches property'' construct and
the "\P{}" negation, ``doesn't match property''.

See ``Unicode Character Properties'' for more details.

You can define your own character properties and use them
in the regular expression with the "\p{}" or "\P{}" construct.

See ``User-Defined Character Properties'' for more details.

•

The special pattern "\X" matches any extended Unicode
sequence--``a combining character sequence'' in Standardese---where the
first character is a base character and subsequent characters are mark
characters that apply to the base character. "\X" is equivalent to
"(?:\PM\pM*)".

•

The "tr///" operator translates characters instead of bytes. Note
that the "tr///CU" functionality has been removed. For similar
functionality see pack('U0', ...) and pack('C0', ...).

•

Case translation operators use the Unicode case translation tables
when character input is provided. Note that "uc()", or "\U" in
interpolated strings, translates to uppercase, while "ucfirst",
or "\u" in interpolated strings, translates to titlecase in languages
that make the distinction.

•

Most operators that deal with positions or lengths in a string will
automatically switch to using character positions, including
"chop()", "chomp()", "substr()", "pos()", "index()", "rindex()",
"sprintf()", "write()", and "length()". An operator that
specifically does not switch is "vec()". Operators that really don't
care include operators that treat strings as a bucket of bits such as
"sort()", and operators dealing with filenames.

•

The "pack()"/"unpack()" letter "C" does not change, since it is often
used for byte-oriented formats. Again, think "char" in the C language.

There is a new "U" specifier that converts between Unicode characters
and code points. There is also a "W" specifier that is the equivalent of
"chr"/"ord" and properly handles character values even if they are above 255.

•

The "chr()" and "ord()" functions work on characters, similar to
"pack("W")" and "unpack("W")", not"pack("C")" and
"unpack("C")". "pack("C")" and "unpack("C")" are methods for
emulating byte-oriented "chr()" and "ord()" on Unicode strings.
While these methods reveal the internal encoding of Unicode strings,
that is not something one normally needs to care about at all.

•

The bit string operators, "& | ^ ~", can operate on character data.
However, for backward compatibility, such as when using bit string
operations when characters are all less than 256 in ordinal value, one
should not use "~" (the bit complement) with characters of both
values less than 256 and values greater than 256. Most importantly,
DeMorgan's laws ("~($x|$y) eq ~$x&~$y" and "~($x&$y) eq ~$x|~$y")
will not hold. The reason for this mathematical faux pas is that
the complement cannot return both the 8-bit (byte-wide) bit
complement and the full character-wide bit complement.

•

lc(), uc(), lcfirst(), and ucfirst() work for the following cases:

•

the case mapping is from a single Unicode character to another
single Unicode character, or

•

the case mapping is from a single Unicode character to more
than one Unicode character.

Things to do with locales (Lithuanian, Turkish, Azeri) do not work
since Perl does not understand the concept of Unicode locales.

See the Unicode Technical Report #21, Case Mappings, for more details.

But you can also define your own mappings to be used in the lc(),
lcfirst(), uc(), and ucfirst() (or their string-inlined versions).

See ``User-Defined Case Mappings'' for more details.

•

And finally, "scalar reverse()" reverses by character rather than by byte.

Unicode Character Properties

Named Unicode properties, scripts, and block ranges may be used like
character classes via the "\p{}" ``matches property'' construct and
the "\P{}" negation, ``doesn't match property''.

For instance, "\p{Lu}" matches any character with the Unicode ``Lu''
(Letter, uppercase) property, while "\p{M}" matches any character
with an ``M'' (mark---accents and such) property. Brackets are not
required for single letter properties, so "\p{M}" is equivalent to
"\pM". Many predefined properties are available, such as
"\p{Mirrored}" and "\p{Tibetan}".

The official Unicode script and block names have spaces and dashes as
separators, but for convenience you can use dashes, spaces, or
underbars, and case is unimportant. It is recommended, however, that
for consistency you use the following naming: the official Unicode
script, property, or block name (see below for the additional rules
that apply to block names) with whitespace and dashes removed, and the
words ``uppercase-first-lowercase-rest''. "Latin-1 Supplement" thus
becomes "Latin1Supplement".

You can also use negation in both "\p{}" and "\P{}" by introducing a caret
(^) between the first brace and the property name: "\p{^Tamil}" is
equal to "\P{Tamil}".

NOTE: the properties, scripts, and blocks listed here are as of
Unicode 5.0.0 in July 2006.

General Category

Here are the basic Unicode General Category properties, followed by their
long form. You can use either; "\p{Lu}" and "\p{UppercaseLetter}",
for instance, are identical.

Single-letter properties match all characters in any of the
two-letter sub-properties starting with the same letter.
"LC" and "L&" are special cases, which are aliases for the set of
"Ll", "Lu", and "Lt".

Because Perl hides the need for the user to understand the internal
representation of Unicode characters, there is no need to implement
the somewhat messy concept of surrogates. "Cs" is therefore not
supported.

Bidirectional Character Types

Because scripts differ in their directionality---Hebrew is
written right to left, for example---Unicode supplies these properties in
the BidiClass class:

For backward compatibility (with Perl 5.6), all properties mentioned
so far may have "Is" prepended to their name, so "\P{IsLu}", for
example, is equal to "\P{Lu}".

Blocks

In addition to scripts, Unicode also defines blocks of
characters. The difference between scripts and blocks is that the
concept of scripts is closer to natural languages, while the concept
of blocks is more of an artificial grouping based on groups of 256
Unicode characters. For example, the "Latin" script contains letters
from many blocks but does not contain all the characters from those
blocks. It does not, for example, contain digits, because digits are
shared across many scripts. Digits and similar groups, like
punctuation, are in a category called "Common".

Block names are given with the "In" prefix. For example, the
Katakana block is referenced via "\p{InKatakana}". The "In"
prefix may be omitted if there is no naming conflict with a script
or any other property, but it is recommended that "In" always be used
for block tests to avoid confusion.

User-Defined Character Properties

You can define your own character properties by defining subroutines
whose names begin with ``In'' or ``Is''. The subroutines can be defined in
any package. The user-defined properties can be used in the regular
expression "\p" and "\P" constructs; if you are using a user-defined
property from a package other than the one you are in, you must specify
its package in the "\p" or "\P" construct.

The subroutines must return a specially-formatted string, with one
or more newline-separated lines. Each line must be one of the following:

•

A single hexadecimal number denoting a Unicode code point to include.

•

Two hexadecimal numbers separated by horizontal whitespace (space or
tabular characters) denoting a range of Unicode code points to include.

•

Something to include, prefixed by ``+'': a built-in character
property (prefixed by ``utf8::'') or a user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.

•

Something to exclude, prefixed by ``-'': an existing character
property (prefixed by ``utf8::'') or a user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.

•

Something to negate, prefixed ``!'': an existing character
property (prefixed by ``utf8::'') or a user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.

•

Something to intersect with, prefixed by ``&'': an existing character
property (prefixed by ``utf8::'') or a user-defined character property,
for all the characters except the characters in the property; two
hexadecimal code points for a range; or a single hexadecimal code point.

For example, to define a property that covers both the Japanese
syllabaries (hiragana and katakana), you can define

sub InKana {
return <<END;
3040\t309F
30A0\t30FF
END
}

Imagine that the here-doc end marker is at the beginning of the line.
Now you can use "\p{InKana}" and "\P{InKana}".

Intersection is useful for getting the common characters matched by
two (or more) classes.

sub InFooAndBar {
return <<'END';
+main::Foo
&main::Bar
END
}

It's important to remember not to use ``&'' for the first set --- that
would be intersecting with nothing (resulting in an empty set).

User-Defined Case Mappings

You can also define your own mappings to be used in the lc(),
lcfirst(), uc(), and ucfirst() (or their string-inlined versions).
The principle is similar to that of user-defined character
properties: to define subroutines in the "main" package
with names like "ToLower" (for lc() and lcfirst()), "ToTitle" (for
the first character in ucfirst()), and "ToUpper" (for uc(), and the
rest of the characters in ucfirst()).

The string returned by the subroutines needs now to be three
hexadecimal numbers separated by tabulators: start of the source
range, end of the source range, and start of the destination range.
For example:

sub ToUpper {
return <<END;
0061\t0063\t0041
END
}

defines an uc() mapping that causes only the characters ``a'', ``b'', and
``c'' to be mapped to ``A'', ``B'', ``C'', all other characters will remain
unchanged.

If there is no source range to speak of, that is, the mapping is from
a single character to another single character, leave the end of the
source range empty, but the two tabulator characters are still needed.
For example:

sub ToLower {
return <<END;
0041\t\t0061
END
}

defines a lc() mapping that causes only ``A'' to be mapped to ``a'', all
other characters will remain unchanged.

(For serious hackers only) If you want to introspect the default
mappings, you can find the data in the directory
$Config{privlib}/unicore/To/. The mapping data is returned as
the here-document, and the "utf8::ToSpecFoo" are special exception
mappings derived from <$Config{privlib}>/unicore/SpecialCasing.txt.
The "Digit" and "Fold" mappings that one can see in the directory
are not directly user-accessible, one can use either the
"Unicode::UCD" module, or just match case-insensitively (that's when
the "Fold" mapping is used).

A final note on the user-defined case mappings: they will be used
only if the scalar has been marked as having Unicode characters.
Old byte-style strings will not be affected.

Character Encodings for Input and Output

See Encode.

Unicode Regular Expression Support Level

The following list of Unicode support for regular expressions describes
all the features currently supported. The references to ``Level N''
and the section numbers refer to the Unicode Technical Standard #18,
``Unicode Regular Expressions'', version 11, in May 2005.

Unicode Encodings

Unicode characters are assigned to code points, which are abstract
numbers. To use these numbers, various encodings are needed.

•

UTF-8

UTF-8 is a variable-length (1 to 6 bytes, current character allocations
require 4 bytes), byte-order independent encoding. For ASCII (and we
really do mean 7-bit ASCII, not another 8-bit encoding), UTF-8 is
transparent.

Note the "A0..BF" in "U+0800..U+0FFF", the "80..9F" in
"U+D000...U+D7FF", the "90..B"F in "U+10000..U+3FFFF", and the
"80...8F" in "U+100000..U+10FFFF". The ``gaps'' are caused by legal
UTF-8 avoiding non-shortest encodings: it is technically possible to
UTF-8-encode a single code point in different ways, but that is
explicitly forbidden, and the shortest possible encoding should always
be used. So that's what Perl does.

As you can see, the continuation bytes all begin with 10, and the
leading bits of the start byte tell how many bytes the are in the
encoded character.

•

UTF-EBCDIC

Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.

•

UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks)

The followings items are mostly for reference and general Unicode
knowledge, Perl doesn't use these constructs internally.

UTF-16 is a 2 or 4 byte encoding. The Unicode code points
"U+0000..U+FFFF" are stored in a single 16-bit unit, and the code
points "U+10000..U+10FFFF" in two 16-bit units. The latter case is
using surrogates, the first 16-bit unit being the high
surrogate, and the second being the low surrogate.

Surrogates are code points set aside to encode the "U+10000..U+10FFFF"
range of Unicode code points in pairs of 16-bit units. The high
surrogates are the range "U+D800..U+DBFF", and the low surrogates
are the range "U+DC00..U+DFFF". The surrogate encoding is

If you try to generate surrogates (for example by using chr()), you
will get a warning if warnings are turned on, because those code
points are not valid for a Unicode character.

Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
itself can be used for in-memory computations, but if storage or
transfer is required either UTF-16BE (big-endian) or UTF-16LE
(little-endian) encodings must be chosen.

This introduces another problem: what if you just know that your data
is UTF-16, but you don't know which endianness? Byte Order Marks, or
BOMs, are a solution to this. A special character has been reserved
in Unicode to function as a byte order marker: the character with the
code point "U+FEFF" is the BOM.

The trick is that if you read a BOM, you will know the byte order,
since if it was written on a big-endian platform, you will read the
bytes "0xFE 0xFF", but if it was written on a little-endian platform,
you will read the bytes "0xFF 0xFE". (And if the originating platform
was writing in UTF-8, you will read the bytes "0xEF 0xBB 0xBF".)

The way this trick works is that the character with the code point
"U+FFFE" is guaranteed not to be a valid Unicode character, so the
sequence of bytes "0xFF 0xFE" is unambiguously ``BOM, represented in
little-endian format'' and cannot be "U+FFFE", represented in big-endian
format".

•

UTF-32, UTF-32BE, UTF-32LE

The UTF-32 family is pretty much like the UTF-16 family, expect that
the units are 32-bit, and therefore the surrogate scheme is not
needed. The BOM signatures will be "0x00 0x00 0xFE 0xFF" for BE and
"0xFF 0xFE 0x00 0x00" for LE.

•

UCS-2, UCS-4

Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
encoding. Unlike UTF-16, UCS-2 is not extensible beyond "U+FFFF",
because it does not use surrogates. UCS-4 is a 32-bit encoding,
functionally identical to UTF-32.

•

UTF-7

A seven-bit safe (non-eight-bit) encoding, which is useful if the
transport or storage is not eight-bit safe. Defined by RFC 2152.

Security Implications of Unicode

•

Malformed UTF-8

Unfortunately, the specification of UTF-8 leaves some room for
interpretation of how many bytes of encoded output one should generate
from one input Unicode character. Strictly speaking, the shortest
possible sequence of UTF-8 bytes should be generated,
because otherwise there is potential for an input buffer overflow at
the receiving end of a UTF-8 connection. Perl always generates the
shortest length UTF-8, and with warnings on Perl will warn about
non-shortest length UTF-8 along with other malformations, such as the
surrogates, which are not real Unicode code points.

•

Regular expressions behave slightly differently between byte data and
character (Unicode) data. For example, the ``word character'' character
class "\w" will work differently depending on if data is eight-bit bytes
or Unicode.

In the first case, the set of "\w" characters is either small---the
default set of alphabetic characters, digits, and the ``_''--or, if you
are using a locale (see perllocale), the "\w" might contain a few
more letters according to your language and country.

In the second case, the "\w" set of characters is much, much larger.
Most importantly, even in the set of the first 256 characters, it will
probably match different characters: unlike most locales, which are
specific to a language and country pair, Unicode classifies all the
characters that are letters somewhere as "\w". For example, your
locale might not think that LATINSMALLLETTERETH is a letter (unless
you happen to speak Icelandic), but Unicode does.

As discussed elsewhere, Perl has one foot (two hooves?) planted in
each of two worlds: the old world of bytes and the new world of
characters, upgrading from bytes to characters when necessary.
If your legacy code does not explicitly use Unicode, no automatic
switch-over to characters should happen. Characters shouldn't get
downgraded to bytes, either. It is possible to accidentally mix bytes
and characters, however (see perluniintro), in which case "\w" in
regular expressions might start behaving differently. Review your
code. Use warnings and the "strict" pragma.

Unicode in Perl on EBCDIC

The way Unicode is handled on EBCDIC platforms is still
experimental. On such platforms, references to UTF-8 encoding in this
document and elsewhere should be read as meaning the UTF-EBCDIC
specified in Unicode Technical Report 16, unless ASCII vs. EBCDIC issues
are specifically discussed. There is no "utfebcdic" pragma or
``:utfebcdic'' layer; rather, ``utf8'' and ``:utf8'' are reused to mean
the platform's ``natural'' 8-bit encoding of Unicode. See perlebcdic
for more discussion of the issues.

Locales

Usually locale settings and Unicode do not affect each other, but
there are a couple of exceptions:

•

You can enable automatic UTF-8-ification of your standard file
handles, default "open()" layer, and @ARGV by using either
the "-C" command line switch or the "PERL_UNICODE" environment
variable, see perlrun for the documentation of the "-C" switch.

•

Perl tries really hard to work both with Unicode and the old
byte-oriented world. Most often this is nice, but sometimes Perl's
straddling of the proverbial fence causes problems.

When Unicode Does Not Happen

While Perl does have extensive ways to input and output in Unicode,
and few other 'entry points' like the @ARGV which can be interpreted
as Unicode (UTF-8), there still are many places where Unicode (in some
encoding or another) could be given as arguments or received as
results, or both, but it is not.

The following are such interfaces. For all of these interfaces Perl
currently (as of 5.8.3) simply assumes byte strings both as arguments
and results, or UTF-8 strings if the "encoding" pragma has been used.

One reason why Perl does not attempt to resolve the role of Unicode in
this cases is that the answers are highly dependent on the operating
system and the file system(s). For example, whether filenames can be
in Unicode, and in exactly what kind of encoding, is not exactly a
portable concept. Similarly for the qx and system: how well will the
'command line interface' (and which of them?) handle Unicode?

Forcing Unicode in Perl (Or Unforcing Unicode in Perl)

Sometimes (see ``When Unicode Does Not Happen'') there are
situations where you simply need to force Perl to believe that a byte
string is UTF-8, or vice versa. The low-level calls
utf8::upgrade($bytestring) and utf8::downgrade($utf8string) are
the answers.

Do not use them without careful thought, though: Perl may easily get
very confused, angry, or even crash, if you suddenly change the 'nature'
of scalar like that. Especially careful you have to be if you use the
utf8::upgrade(): any random byte string is not valid UTF-8.

Using Unicode in XS

If you want to handle Perl Unicode in XS extensions, you may find the
following C APIs useful. See also ``Unicode Support'' in perlguts for an
explanation about Unicode at the XS level, and perlapi for the API
details.

•

"DO_UTF8(sv)" returns true if the "UTF8" flag is on and the bytes
pragma is not in effect. "SvUTF8(sv)" returns true is the "UTF8"
flag is on; the bytes pragma is ignored. The "UTF8" flag being on
does not mean that there are any characters of code points greater
than 255 (or 127) in the scalar or that there are even any characters
in the scalar. What the "UTF8" flag means is that the sequence of
octets in the representation of the scalar is the sequence of UTF-8
encoded code points of the characters of a string. The "UTF8" flag
being off means that each octet in this representation encodes a
single character with code point 0..255 within the string. Perl's
Unicode model is not to use UTF-8 until it is absolutely necessary.

•

"uvuni_to_utf8(buf, chr)" writes a Unicode character code point into
a buffer encoding the code point as UTF-8, and returns a pointer
pointing after the UTF-8 bytes.

•

"utf8_to_uvuni(buf, lenp)" reads UTF-8 encoded bytes from a buffer and
returns the Unicode character code point and, optionally, the length of
the UTF-8 byte sequence.

•

"utf8_length(start, end)" returns the length of the UTF-8 encoded buffer
in characters. "sv_len_utf8(sv)" returns the length of the UTF-8 encoded
scalar.

•

"sv_utf8_upgrade(sv)" converts the string of the scalar to its UTF-8
encoded form. "sv_utf8_downgrade(sv)" does the opposite, if
possible. "sv_utf8_encode(sv)" is like sv_utf8_upgrade except that
it does not set the "UTF8" flag. "sv_utf8_decode()" does the
opposite of "sv_utf8_encode()". Note that none of these are to be
used as general-purpose encoding or decoding interfaces: "use Encode"
for that. "sv_utf8_upgrade()" is affected by the encoding pragma
but "sv_utf8_downgrade()" is not (since the encoding pragma is
designed to be a one-way street).

•

is_utf8_char(s) returns true if the pointer points to a valid UTF-8
character.

•

"is_utf8_string(buf, len)" returns true if "len" bytes of the buffer
are valid UTF-8.

•

"UTF8SKIP(buf)" will return the number of bytes in the UTF-8 encoded
character in the buffer. "UNISKIP(chr)" will return the number of bytes
required to UTF-8-encode the Unicode character code point. "UTF8SKIP()"
is useful for example for iterating over the characters of a UTF-8
encoded buffer; "UNISKIP()" is useful, for example, in computing
the size required for a UTF-8 encoded buffer.

•

"utf8_distance(a, b)" will tell the distance in characters between the
two pointers pointing to the same UTF-8 encoded buffer.

•

"utf8_hop(s, off)" will return a pointer to an UTF-8 encoded buffer
that is "off" (positive or negative) Unicode characters displaced
from the UTF-8 buffer "s". Be careful not to overstep the buffer:
"utf8_hop()" will merrily run off the end or the beginning of the
buffer if told to do so.

•

"pv_uni_display(dsv, spv, len, pvlim, flags)" and
"sv_uni_display(dsv, ssv, pvlim, flags)" are useful for debugging the
output of Unicode strings and scalars. By default they are useful
only for debugging---they display all characters as hexadecimal code
points---but with the flags "UNI_DISPLAY_ISPRINT",
"UNI_DISPLAY_BACKSLASH", and "UNI_DISPLAY_QQ" you can make the
output more readable.

•

"ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2)" can be used to
compare two strings case-insensitively in Unicode. For case-sensitive
comparisons you can just use "memEQ()" and "memNE()" as usual.

For more information, see perlapi, and utf8.c and utf8.h
in the Perl source code distribution.

BUGS

Interaction with Locales

Use of locales with Unicode data may lead to odd results. Currently,
Perl attempts to attach 8-bit locale info to characters in the range
0..255, but this technique is demonstrably incorrect for locales that
use characters above that range when mapped into Unicode. Perl's
Unicode support will also tend to run slower. Use of locales with
Unicode is discouraged.

Interaction with Extensions

When Perl exchanges data with an extension, the extension should be
able to understand the UTF8 flag and act accordingly. If the
extension doesn't know about the flag, it's likely that the extension
will return incorrectly-flagged data.

So if you're working with Unicode data, consult the documentation of
every module you're using if there are any issues with Unicode data
exchange. If the documentation does not talk about Unicode at all,
suspect the worst and probably look at the source to learn how the
module is implemented. Modules written completely in Perl shouldn't
cause problems. Modules that directly or indirectly access code written
in other programming languages are at risk.

For affected functions, the simple strategy to avoid data corruption is
to always make the encoding of the exchanged data explicit. Choose an
encoding that you know the extension can handle. Convert arguments passed
to the extensions to that encoding and convert results back from that
encoding. Write wrapper functions that do the conversions for you, so
you can later change the functions when the extension catches up.

To provide an example, let's say the popular Foo::Bar::escape_html
function doesn't deal with Unicode data yet. The wrapper function
would convert the argument to raw UTF-8 and convert the result back to
Perl's internal representation like so:

Sometimes, when the extension does not convert data but just stores
and retrieves them, you will be in a position to use the otherwise
dangerous Encode::_utf8_on() function. Let's say the popular
"Foo::Bar" extension, written in C, provides a "param" method that
lets you store and retrieve data according to these prototypes:

Some extensions provide filters on data entry/exit points, such as
DB_File::filter_store_key and family. Look out for such filters in
the documentation of your extensions, they can make the transition to
Unicode data much easier.

Speed

Some functions are slower when working on UTF-8 encoded strings than
on byte encoded strings. All functions that need to hop over
characters such as length(), substr() or index(), or matching regular
expressions can work much faster when the underlying data are
byte-encoded.

In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
a caching scheme was introduced which will hopefully make the slowness
somewhat less spectacular, at least for some operations. In general,
operations with UTF-8 encoded strings are still slower. As an example,
the Unicode properties (character classes) like "\p{Nd}" are known to
be quite a bit slower (5-20 times) than their simpler counterparts
like "\d" (then again, there 268 Unicode characters matching "Nd"
compared with the 10 ASCII characters matching "d").

Porting code from perl-5.6.X

Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
was required to use the "utf8" pragma to declare that a given scope
expected to deal with Unicode data and had to make sure that only
Unicode data were reaching that scope. If you have code that is
working with 5.6, you will need some of the following adjustments to
your code. The examples are written such that the code will continue
to work under 5.6, so you should be safe to try them out.

•

A filehandle that should read or write UTF-8

if ($] > 5.007) {
binmode $fh, ":encoding(utf8)";
}

•

A scalar that is going to be passed to some extension

Be it Compress::Zlib, Apache::Request or any extension that has no
mention of Unicode in the manpage, you need to make sure that the
UTF8 flag is stripped off. Note that at the time of this writing
(October 2002) the mentioned modules are not UTF-8-aware. Please
check the documentation to verify if this is still true.

If you believe the scalar comes back as UTF-8, you will most likely
want the UTF8 flag restored:

if ($] > 5.007) {
require Encode;
$val = Encode::decode_utf8($val);
}

•

Same thing, if you are really sure it is UTF-8

if ($] > 5.007) {
require Encode;
Encode::_utf8_on($val);
}

•

A wrapper for fetchrow_array and fetchrow_hashref

When the database contains only UTF-8, a wrapper function or method is
a convenient way to replace all your fetchrow_array and
fetchrow_hashref calls. A wrapper function will also make it easier to
adapt to future enhancements in your database driver. Note that at the
time of this writing (October 2002), the DBI has no standardized way
to deal with UTF-8 data. Please check the documentation to verify if
that is still true.